THE HAGUE, March 25 (Reuters) - World leaders called on Tuesday for countries to cut their stocks of highly enriched nuclear fuel to the minimum to help prevent al Qaeda-style militants from obtaining material for atomic bombs.

Winding up a third nuclear security summit since 2010 and one overshadowed by the Ukraine crisis, leaders from 53 countries - including U.S. President Barack Obama - said much headway had been made in the past four years.

But they also underlined that many challenges remained and stressed the need for increased international cooperation to make sure highly enriched uranium (HEU), plutonium and other radioactive substances do not fall into the wrong hands.

The United States and Russia set aside their differences over Crimea to endorse the meeting's final statement aimed at enhancing nuclear security around the world, together with other big powers including China, France, Germany and Britain.

"We encourage states to minimise their stocks of HEU and to keep their stockpile of separated plutonium to the minimum level, both as consistent with national requirements," said the communique, which went further in this respect than the previous summit, in Seoul in 2012.

A fourth summit will be held in Chicago in 2016. The summit process began in Washington in 2010.

Obama told the summit's final session: "You've set a high bar on what needs to be done in Chicago ... I think it's important for us not to relax but to accelerate our work over the next two years."

To drive home the message of the importance of being prepared, the Dutch hosts sprang a surprise by organising an simulation game for the leaders in which they were asked to react to a fictitious nuclear attack or accident in a made-up state, officials said.

Analysts say that radical groups could theoretically build a crude but deadly nuclear bomb if they had the money, technical knowledge and fissile substances needed.

Obtaining weapons-grade nuclear material - HEU or plutonium - poses the biggest challenge for militant groups, so it must be kept secure both at civilian and military sites, they say.

Around 2,000 metric tons of highly-radioactive materials are spread across hundreds of sites in 25 countries. Most of the materials is under military control but a significant quantity is stored in less secured civilian locations, according to the Fissile Materials Working Group (FMWG).

"MODERATE STEPS"

Referring to a push to use low-enriched uranium (LEU) as fuel in research and other reactor types instead of the more proliferation-prone HEU, the summit statement said: "We encourage states to continue to minimise the use of HEU through the conversion of reactor fuel from HEU to LEU, where technically and economically feasible.

"Similarly, we will continue to encourage and support efforts to use non-HEU technologies for the production of radio-isotopes, including financial incentives," it said.

An apple-sized amount of plutonium in a nuclear device and detonated in a highly populated area could instantly kill or wound hundreds of thousands of people, according to the Nuclear Security Governance Experts Group (NSGEG) lobby group.

But a so-called "dirty bomb" is seen as a more likely threat than an atomic bomb: conventional explosives are used to disperse radiation from a radioactive source, which can be found in hospitals or other places that may not be very well secured.

In December, Mexican police found a truck they suspected was stolen by common thieves and which carried a radioactive medical material that could have provided such an ingredient.

The FMWG, an international group of over 70 security experts, said the summit had taken "moderate steps" toward stopping dangerous weapons-usable nuclear materials from going astray but that bolder and more concerted action is needed.

"Today's nuclear security system - a hodgepodge of voluntary national pledges without global standards to lock down nuclear materials - needs more than just patching up to prevent a nuclear terrorist attack," the FMWG said in a statement. (Editing by Mark Heinrich)

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